1. Field of the Invention
The present invention relates to a magnetostrictive load sensor that electromagnetically detects a load using a magnetostrictive effect and a movable object including the same.
2. Description of the Related Art
Load sensors used for movable objects such as motorcycles, water bikes, mobile racks, transport equipment, power-assisted bicycles, or electric wheelchairs are required to be miniaturized. Magnetostrictive load sensors have been put to practical use as small-sized load sensors.
The magnetostrictive load sensors convert changes in magnetic properties of members to which loads are applied into changes in voltages and detect the loads on the basis of the changes in the voltages.
JP 11-241955 A discloses a load detecting device serving as a magnetostrictive load sensor. The load detecting device disclosed in JP 11-241955A includes rod-shaped magnetic body, an exciting coil, a detecting coil, and a magnetic shield case.
The rod-shaped magnetic body is formed of a magnetic material. The exciting coil and the detecting coil are wound around the magnetic body with the coils electrically insulated from each other. The magnetic shield case is formed of a magnetic material, and accommodates the magnetic body, the exciting coil, and the detecting coil.
One end of the magnetic body projects upward through a hole provided at the top of the magnetic shield case. A joint is provided at an upper end of the magnetic body.
The load is applied to the magnetic body through a joint with the magnetic body magnetized by the exciting coil. This causes the magnetic body to be compressed when the load is applied in the axial direction of the magnetic body. As a result, the impedance of the load detecting device changes by an inverse magnetostrictive effect, and a voltage across both ends of the detecting coil changes. Therefore, the load applied to the magnetic body is calculated on the basis of the change in the voltage in the detecting coil.
In the load detecting device disclosed in JP 11-241955 A described above, however, the magnetic shield case formed of a magnetic material supports a lower end of the rod-shaped magnetic body and covers the respective outer peripheries, tops, and bottoms of the exciting coil and the detecting coil. This causes the magnetic shield case to function as a magnetic path when the magnetic body is magnetized by the exciting coil.
The magnetic shield case supports a lower end of the magnetic body. When the load is applied to the upper end of the magnetic body, therefore, a stress is also exerted on an abutment portion of the lower end of the magnetic body and the magnetic shield case.
When the direction of the load applied to the magnetic body is shifted from the axial direction of the magnetic body, a stress distribution at the lower end of the magnetic body becomes non-uniform, so that a stress concentrated portion occurs. When the direction of the load applied to the magnetic body differs, the position and the size of the stress concentrated portion occurring at the lower end of the magnetic body also differ. Even if the load applied to the magnetic body is fixed, therefore, the magnetic properties at the lower end of the magnetic body vary depending on the position and the size of the occurring stress concentrated portion.
Furthermore, the lower end of the magnetic body and the magnetic shield case are abutted against each other. When the stress concentrated portion occurs at the lower end of the magnetic body, therefore, a stress concentrated portion corresponding to the position and the size of the stress concentrated portion occurring in the magnetic body also occurs in the abutment portion of the magnetic shield case and the magnetic body. In the magnetic shield case, therefore, the magnetic properties also vary depending on the position and the size of the stress concentrated portion occurring in the magnetic body.
When the direction of the load applied to the magnetic body differs, the change in the voltage across both the ends of the detecting coil vary under the effects of the variation in the magnetic properties at the lower end of the magnetic body and the variation in the magnetic properties of the magnetic shield case. As a result, the variation in the output of the load detecting device increases, so that the reliability of the load detecting device is degraded.
Furthermore, even when the direction of the load applied to the magnetic body coincides with the axial direction of the magnetic body, the stress distribution at the lower end of the magnetic body becomes non-uniform, so that the stress concentrated portion occurs when errors only slightly occur in the shape and the size of the abutment portion of the magnetic body and the magnetic shield case.
When load detecting devices, described above, are mass-produced, the voltage across both the ends of the detecting coil varies between the load detecting devices due to the errors in the shape and the size of the abutment portion of the magnetic body and the magnetic shield case. As a result, the manufacturing yield of the load detecting device is reduced.